Apparatus and method for electrical heating of aircraft skin for background matching

Abstract

Apparatus and method for background matching of an aircraft or spacecraft which includes the direct electrical heating of the aircraft skin to minimize a negative infrared contrast signature detected from above platform infrared detectors. An infrared detector on the bottom of the aircraft detects the background radiance which is compared with the aircraft radiance determined from the power flight conditions or from direct sensors. An on board computer compares the background and aircraft radiance to determine the amount of heat required to match the background. The computer controls a power source that heats an electrical resistor within the aircraft skin to increase the skin temperature. Selective heating of portions of the aircraft skin may be provided by a plurality of resistors strategically placed on the aircraft fuselage.

I claim: 
     
       1. A system for matching the radiance of the upper skin of an aircraft to the background radiance emanating from below the aircraft, said system comprising: first means for sensing the radiance of the aircraft upper skin;   second means for sensing the background radiance emanating from below the aircraft;   electrical resistor means coupled to the aircraft skin for directly heating the aircraft upper skin;   control means coupled to said first and second sensing means for comparing the background radiance with the aircraft upper skin radiance and for determining the magnitude of the electrical power required to heat the upper aircraft skin sufficient to increase the aircraft radiance to match the background radiance, said control means producing a control signal for generating the required electrical power; and   electrical power means coupled to said control means for providing said required magnitude of electrical power to said electrical resistor means in response to said control signal thereby matching the aircraft radiance with said background radiance.   
     
     
       2. The system of claim 1 wherein said first means for sensing the aircraft upper skin radiance includes a navigation system for said aircraft. 
     
     
       3. The system of claim 1 wherein said first means for sensing the aircraft upper skin radiance includes a thermocouple coupled to the aircraft upper skin for directly sensing the radiance of said aircraft upper skin. 
     
     
       4. The system of claim 1 wherein said second means for sensing the background radiance includes an infrared detector located on the lower skin of the aircraft for producing an electrical output signal representative of the background radiance. 
     
     
       5. The system of claim 1 wherein said control means includes a computer. 
     
     
       6. The system of claim 1 wherein said electrical resistor means is secured to the bottom of the aircraft upper skin made of metallic materials. 
     
     
       7. The system of claim 1 wherein said electrical resistor means is embedded within the aircraft upper skin made of composite materials. 
     
     
       8. The system of claims 6 or 7 wherein said electrical resistor means is a wire mesh. 
     
     
       9. The system of claim 1 further including a plurality of electrical resistor means coupled to said aircraft upper skin at a plurality of detection locations. 
     
     
       10. The system of claim 9 wherein said means for sensing the aircraft skin radiance includes a plurality of thermal sensors coupled to said aircraft skin proximate to said electrical resistor means. 
     
     
       11. The system of claim 10 wherein said control means compares the background radiance with the aircraft skin radiance at each detection location and produces a control signal on a plurality of output channels to generate the required electrical power to each of said electrical resistor means. 
     
     
       12. A system for matching the radiance of the upper skin of an aircraft to the background radiance emanating from below the aircraft, said system comprising: means for determining the radiance of said aircraft skin while in flight;   means for determining the background radiance emanating from below said aircraft while in flight;   means for comparing the background radiance with the aircraft skin radiance and producing an output signal representative of a negative contrast signature, said negative contrast signature being present when the aircraft radiance is less than said background radiance;   means for electrically heating the aircraft skin to increase the radiance thereof;   means for determining the magnitude of the electrical power required to heat the aircraft skin sufficient to increase the aircraft radiance to match the background radiance, and   means for providing said required magnitude of electrical power to said heating means thereby resulting in a substantially zero contrast signature.   
     
     
       13. The system of claim 12 wherein said means for determining the aircraft skin radiance includes a first processor means coupled to the aircraft navigation system for computing the aircraft skin radiance from power flight conditions determined by said navigation system and for producing output data representative of said aircraft skin radiance. 
     
     
       14. The system of claim 12 wherein said means for determining the aircraft skin radiance includes means coupled to the upper skin of said aircraft for directly sensing the radiance of said aircraft upper skin. 
     
     
       15. The system of claim 14 wherein said means for directly sensing the radiance of the aircraft upper skin includes a thermal sensor coupled to a first processor. 
     
     
       16. The system of claims 13 or 15 wherein said means for determining the background radiance includes an infrared detector located on the lower skin of the aircraft for producing an electrical output signal representative of the background radiance. 
     
     
       17. The system of claim 16 further including a second processor means coupled to said infrared detector for converting said electrical output signal to output data. 
     
     
       18. The system of claim 17 wherein said comparing means includes a comparator coupled to said first and second processors for comparing the output data from each of said first and second processor and for producing an output signal in response to the aircraft skin radiance being less than said background radiance. 
     
     
       19. The system of claim 18 wherein said means for determining the magnitude of the electrical power required to heat the aircraft skin includes a third processor means responsive to the output signal of said comparator for computing the required electrical power and producing a control signal in response to said comparator output signal. 
     
     
       20. The system of claim 19 wherein said means for producing the magnitude of electrical power to said heating means includes an existing aircraft power supply coupled to said third processor means and said heating means. 
     
     
       21. The system of claims 12 or 20 wherein said means for electrically heating said aircraft skin includes an electrical resistor means. 
     
     
       22. The system of claim 21 wherein said electrical resistor means is secured to the bottom of the aircraft skin made of metallic materials. 
     
     
       23. The system of claim 21 wherein said electrical resistor means is embedded within the aircraft skin made of composite materials. 
     
     
       24. The system of claim 22 wherein said electrical resistor is a wire mesh. 
     
     
       25. The system of claim 23 wherein said electrical resistor is a wire mesh. 
     
     
       26. The system of claim 12 wherein said means for heating the aircraft skin includes a plurality of electrical resistor means coupled to said aircraft skin at a plurality of detection locations. 
     
     
       27. The system of claim 26 wherein said means for determining the aircraft skin radiance includes a plurality of thermal sensors coupled to said aircraft skin proximate to said electrical resistor means. 
     
     
       28. The system of claim 27 wherein said means for comparing the background radiance with the aircraft radiance includes a comparator coupled to each of said thermal sensors for comparing the background radiance with the aircraft skin radiance at each location as measured by each thermal sensor and for producing on a plurality of output channels an output signal in response to the existence of a negative contrast signature at each of said detection locations. 
     
     
       29. The system of claim 28 wherein said means for determining the magnitude of the electrical power required to heat the aircraft skin includes a processor means responsive to the plurality of output signals of said comparator for computing the required electrical power for each of said detection locations and for producing a control signal on a plurality of corresponding output channels for generating the required electrical power to each of said electrical resistor means. 
     
     
       30. A method for matching the radiance of the upper skin of an aircraft to the background radiance emanating from below said aircraft, comprising the steps of: determining the radiance of the aircraft skin;   determining the background radiance emanating from below said aircraft,   comparing the background radiance with the aircraft skin radiance and determining the presence of a negative contrast signature, said negative contrast signature being present when said aircraft radiance is less than said background radiance;   determining the electrical power required to heat the aircraft skin sufficient to increase the radiance thereof to match the background radiance; and   electrically heating the aircraft skin with the required electrical power resulting in a substantially zero contrast signature.   
     
     
       31. The method of claim 30 including the steps of computing the aircraft skin radiance from power flight conditions taken from the aircraft navigation system. 
     
     
       32. The method of claim 30 including providing said required electrical power to an electrical resistor means coupled to said aircraft skin for heating the aircraft skin. 
     
     
       33. The method of claim 32 including providing a plurality of electrical resistor means for selectively heating a plurality of detection locations. 
     
     
       34. The method of claim 33 including providing a plurality of thermal sensors coupled to said aircraft skin for determining the aircraft skin radiance at said plurality of detection locations. 
     
     
       35. The method of claim 34 including comparing the background radiance with the aircraft skin radiance of each detection location and determining the presence of a negative contrast signature at each said detection location. 
     
     
       36. The method of claim 35 including computing the electrical power required to heat each detection location to increase the radiance thereof to match the background radiance. 
     
     
       37. The method of claim 36 including selectively providing the required electrical power to each of said electrical resistor means. 
     
     
       38. The method of claim 30 wherein the electrical power is determined in accordance with the equation: ##EQU20## where, h T  =Sum of the convective and radiative heat transfer coefficients T r  =Recovery temperature which is a function of aircraft altitude and Mach number   A=Surface area to be heated   η=Conversion efficiency from mechanical to electrical and from electrical to thermal power   Po=Dimensionless power ratio.   
     
     
       39. The method of claim 38 wherein the dimensionless power ratio is determined in accordance with the equation: ##EQU21## where, Γ=Reduction in detection range ratio, and E n  =Dimensionless energy ratio.   
     
     
       40. The method of claim 39 wherein the reduction in detection range ratio is determined in accordance with the equation: ##EQU22## where, R=Observer's range capability with aircraft electrical power on R o  =Observer's range capability without aircraft electrical power.   
     
     
       41. The method of claim 40 wherein the dimensionless energy ratio is determined in accordance with the equation: ##EQU23## where, G n  =Target (or aircraft) projected area (projected in the observers field of view) T r  =Recovery temperature   τ.sub. = Average atmospheric transmittance ##EQU24## C 1  =Planck's first constant C 2  =Planck's second constant   L B ,λ =Spectral background radiance   L F ,λ =Spectral foreground radiance   ε.sub.λ =Spectral emissivity of the target ##EQU25##